Australia's top power producer AGL Energy on Monday turned down a A$250 million ($188 million) offer for an ageing coal-fired energy plant from Chinese-owned Alinta Energy and shunned government pleas to keep the facility open beyond 2022. AGL's Liddell power plant, which supplies 10 percent of the electricity used in Australia's most populous state, New South Wales, is seen as crucial for energy security as the grid becomes more reliant on intermittent wind and solar power. A sale to Alinta would have kept the plant open for longer and increased competition in the market, keeping a lid on soaring electricity prices - a hot political potato with state and federal elections looming in the next 12 months.